Typically, the nodes of an ad hoc network are randomly and arbitrarily deployed in such a manner that their locations are not necessarily known a priori. For example, wireless sensors are often deployed in ad hoc networks and are used for sensing, detection, and tracking. Such networks are being applied in military, ecological, environmental, and domestic systems for monitoring and/or control. Wireless sensor networks are also being used to automate buildings, universities, factories, etc. Because of the ad hoc nature of these networks, few assumptions can be made about their topologies.
Further, in some cases, the nodes of a network can change locations with varying or constant frequency. A network comprised of such mobile nodes is typically referred to as a mobile network.
Most location estimation systems locate the nodes in mobile and ad hoc networks by trilaterating and/or triangulating signals received by the nodes in order to obtain an estimate of the node's position.
A LOS (line-of-sight) path or direct path is the straight line connecting the transmitter and the receiver. NLOS (non-line-of-sight) signals occur due to multi-path conditions in which the received signals have followed reflected, diffracted, and/or scattered paths. Such signals introduce excess path lengths in the actual Euclidian distance between the transmitting nodes and the receiving nodes. Thus, an NLOS error is introduced in the trilateration and/or triangulation and is defined to be the excess distance traversed compared to the distance traversed along the direct path. This excess distance is always positive. The corruption of LOS signals by NLOS signals and also by Gaussian measurement noise are the major sources of error in all location estimation systems.
The Global Positioning System (GPS) is perhaps the most widely publicized location-sensing system. Unfortunately, GPS does not scale well in dense urban areas or in indoor locations.
Also, modeling of the radio propagation environment helps in providing a more accurate location estimate by mitigating the effect of NLOS errors. While reasonably accurate radio propagation models exist for outdoor conditions, there are unfortunately no such unanimously accepted models for indoor environments. Attempts have been made to mitigate the effects of NLOS errors. However, in the absence of a suitable model for predicting the location of a mobile terminal, it is possible that the node may be far away from its estimated location.
Therefore, rather than implementing location prediction as described above, the problem of discovering the location of a node might instead be considered in terms of finding the geographical region in which a node is guaranteed to be found and of then reducing or minimizing the size of this region.
It can be assumed that a small percentage of the terminals (nodes) in the network know their locations with a high degree of accuracy—such as by using GPS or by some other means. Such nodes may be termed reference nodes. A distributed algorithm can implement computational geometric techniques in order to compute the smallest region within which a node is guaranteed to be found, based on all non-reference nodes in the network.
In addition, the location of the regions containing the nodes in the network can be improved through the exchange of location information between the neighbor nodes in O(nD) time, where n and D are the number of nodes and diameter of the network, respectively.
The present invention, therefore, is directed to a system and/or method for finding the geographical region in which a node is guaranteed to be found. The present invention, for example, can implement one or more of the features discussed above, such as minimizing the size the a region in which a node is located.